Issue:

Analysis:

When you try to telnet the HUB server IP on port 25 you receive this error:

4.3.1 Insufficient system resources

On an SMTP server on the front of the HUB server –that should deliver emails to the HUB server-, the below warning is logged in the Application Event log:

Message delivery to the host ‘HUB_SERVER_IP’ failed while delivering to the remote domain ‘DOMAIN.TLD’ for the following reason: An SMTP protocol error occurred. The SMTP verb which caused the error is ‘MAIL’. The response from the remote server is ‘452 4.3.1 Insufficient system resources

Well, now go to the HUB server, open the event viewer, and filtered the Application log by:

Source: MSExchangeTransport

Category: ResourceManager

You will find Warning(s) error(s) like the one mentioned below:

Cause:

That’s due to the Back Pressure effect of Exchange 2007/2010, that checks periodically on some aspects on the HUB server(s) and see if it’s out of the limit or not –in my case above, there’s an issue with the free disk space on the Partition C:\-.

Solution:

Solve the issue mentioned in the event viewer.

Note: While it’s possible to disable the Back Pressure, it’s not recommended at all.

best way to go, is to investigate why it’s happening, and solve the root cause.